The present invention relates to lithium manganese oxide intercalation compounds useful as active electrode materials in Li-ion rechargeable batteries and, particularly, to oxy-fluoride complexes of magnesium-substituted manganese oxides and their use to improve the cycling stability and capacity of such batteries.
Lithium manganese oxide intercalation compounds, nominally LiMn.sub.2 O.sub.4, have been increasingly proven to be effective and economical materials for the fabrication of secondary, rechargeable Li-ion electrolytic cells and composite batteries. Successful batteries of this type are described in U.S. Pat. Nos. 5,296,318 and 5,460,904. These batteries exhibit an admirable level of electrical storage capacity and recharge cycling stability over a wide range of voltages; however, these properties have not been considered entirely satisfactory to meet the increasingly stringent requirements of modern electronic equipment and applications.
Extensive investigations have been undertaken to improve these noted properties, and such works have resulted in determinations that variations in the structural parameters of the LiMn.sub.2 O.sub.4 spinel, for example, the a-axis lattice dimension of the compound, contribute significantly to ultimate cell performance. Such structural parameters have in turn been found to depend to a great extent upon the constitution of the intercalation compound and upon the conditions of its synthesis. Recently, in U.S. Ser. No. 08/706,546, filed Sep. 6, 1996, now U.S. Pat. No. 5,675,645, incorporated herein by reference, Amatucci at el. achieved extraordinary improvement in capacity capabilities and cycling stability through the anionic fluorine substitution of lithium manganese oxide intercalation electrode compounds, including such compounds in which the manganese oxides were cationically-substituted in part with transition metal ions.
Further investigations into other cationic substitutions in the fluoro-substituted lithium manganese oxide structure led to the improved battery capacity and cycling characteristics, particularly at elevated temperature, described by Amatucci in U.S. Ser. No. 08/868,553, filed Jun. 4, 1997. There the substitution of a portion of the manganese with lighter aluminum, as in a general formula Li.sub.1+x Al.sub.y Mn.sub.2-x-y O.sub.4-z F.sub.z, effected a distinct improvement in the theoretical capacity of resulting electrolytic cells, and contributed significantly to the stabilization of such capacity over extended high-temperature cycling. The present invention derives from the yet further discovery that the cationic substitution of magnesium in oxy-fluoride lithium manganese oxide intercalation compounds, as in a general formula Li.sub.1+x Mg.sub.y Mn.sub.2-x-y O.sub.4-z F.sub.z, profoundly improves the stability of cell capacity during long-term recharge cycling in elevated temperature ranges, i.e., about 55.degree. C.